The present invention relates to an improved surface treatment for an iron-based permanent magnet including rare-earth elements and, more particularly, to a permanent magnet having high resistance to mechanical, thermal and/or chemical attack, and also having excellent magnetic properties.
Permanent magnet materials are very important electric and/or electronic materials which are incorporated in various items such as consumer electronic equipment used in homes, motor vehicles and peripheral terminal devices for computers.
Recently, the tendency to reduce in size and to enhance the efficiency of the equipment in which permanent magnets are used has necessitated a further improvement in the characteristics of the permanent magnet materials employed.
A new type of high efficiency magnet, which does not include highly expensive samarium and cobalt, is the R--Fe--B type (where R is one element or a mixture of rare-earth elements including yttrium).
This type of magnet exhibits an extremely high energy product, or (BH) max, of 25 MGOe or more. It includes light rare-earth elements such as Nd and/or Pr and Fe as its main constituent element.
A sintered magnet of Nd--Fe--B is disclosed in European Laid Open Patent Application 0101552 A.
A bulk magnet of Nd--Fe--B type made from rapidly quenched metal flakes (Rapidly Quenched Type of Magnet, hereinafter) is disclosed in European Laid Open Patent Application 0108474 A.
Although these magnets have magnetic anisotropy and a high energy product of 25 MGOe or more, they are inferior to the previous samarium-cobalt type of magnets with respect to corrosion resistance, because they include rare earth elements and iron, both of which are easily oxidized to cause, for example, red rust on their surface. The efficiency of the magnet and the uniformity of magnetic properties decrease when this magnet corrodes.
In order to prevent corrosion of these Nd--Fe--B type of magnets, various methods have been proposed to produce a corrosion resistive layer on their surfaces.
The idea of coating a resin layer on a surface of magnet material is disclosed in Japan Laid Open Patent Application 6377104. Unfortunately, however, the protective layer of resin is generally inferior to a metal plating layer. Because the resin layer is mechanically weaker than a metal plating layer, a scratch is easily made on the resin layer during handling when the resin coated magnet is being inserted into electric equipment. The scratched portion of the resin layer becomes a starting point through which the magnet and the magnet material can be attacked. Even if the resin remains intact, it is inferior to a metal plating layer, because the resin layer is very hygroscopic. It is thus preferable to coat the surface of a magnet body with a metal plating film.
A nickel-plated layer is superior to an oxygen-resist resin layer because the nickel layer itself is mechanically stronger and much less hygroscopic than a layer of resin.
However, it is a problem that a nickel-plated layer typically has many pin holes in contrast to an oxygen-resist resin layer. Because of these pin holes, a magnet body covered by the nickel layer is exposed to corrosion over time by water penetrating through the pin holes to the magnet body, even though the nickel layer itself is not hygroscopic.
The reliability of the corrosion resistive film is reduced if it cannot prevent attack on the substrate through pin-holes. When the substrate is attacked through pin-holes, corroded portions are produced on a magnet body under the protective film, even though a visual inspection indicates that the corrosion resistive film still seems to cover the surface of the magnet. It is important to compensate for the defects in the corrosion resistive metal film to protect the magnet surface.